Analysis techniques based on nuclear magnetic resonance



- ANALYSIS TECHNIQUES BASED on NUCLEAR MAGNETIC RESONANCE Filed July 27,1956 R- J. RUBLE 3 Sheets-Sheeil flw/U liu WU 4 MZ /fi 9 Z M 0 m i f, 3/mm- 1|} V nu WW UHF |l.H.| m bur 4/w// a m n BL fi yr A z A e v 1: 5m 0z a 7 l x 4/ L w 2 5 Z Au ll. m f we m oww 7% 5 M 1 Nov. 10, 1959 Nov.10; 1959 R. J. RUBLE' ,9

,ANALYSIS TECHNIQUES BASED ON NUCLEAR MAGNETIC RESONANCE Filed Jul 27,1956 :s Sheets-Sheet z Tiuzrzio.

OURCE i 44 43 4? a c, L E Pam's? v 33 2 4 wig/m5 Tim/A e45 05044470?Mam/447m J2 AVE A 1/5 gave-09f flvrsgmrae yam/2E Nov. 10, 1959 R. J.RUBLE 2,912,641

ANALYSIS TECHNIQUES BASED ON NUCLEAR MAGNETIC RESONANCE Filed July 27,1956 3 Sheets-Sheet 3 TECHNIQUES BASED ON NUCLEAR MAGNETIC RESONANCERaymond J. Ruble, Dobbs Ferry, N.Y., assignor to Texaco DevelopmentCorporation, New York, N.Y., a corporation of Delaware Application July27, 1956, Serial No. 600,580

. 22 Claims. (Cl. 324.5)

ANALYSIS abandoned) and contains substantially the same disclosure asSerial No. 357,95l.

As is known it is possible to use these nuclear resonance analysistechniques to obtain two kinds of information: (1) information as towhether a sample contains any of a number of elements which are capableof exhibiting nuclear resonance, and (2) information as to the kind(s)of molecules in which any such nuclei which may be present are bound.For example, as is disclosed in one or more of several copendingapplications which are assigned "to the assignee hereof; i.e.,applications: Serial No. 238,754, filed July 26, 1951 (now abandoned);Serial No. 368,547, filed July 17, 1953 (a continuation case of238,754); Serial No. 352,559, filed May 1, 1953 and Serial No. 353,746,filed May 8, 1953, it is possible to use techniques based on nuclearmagnetic resonance to determine in situ whether or not a sample obtainedfrom or comprised in an earth stratum traversed by a bore hole containsthe particular element, hydrogen, and, if it does, whether the hydrogenis a constituent thereof as part of a hydrocarbon substance or of water.

Two essential steps are involved in obtaining the first kind ofinformation: (1) the sample must be initially magnetized by a relativelystrong uni-directional mag netic field and (2) it must then be permeatedby a relatively weak magnetic field of transverse orientation whichrapidly alternates at exactly the rate at which nuclei, Whose presenceit is sought to determine, will be gyromagnetically precessing in theuni-directional field for any given intensity thereof.

Because of this an oscillator which is used for supplying the energy fortransversely permeating the sample must be adjusted so that, for a givenmodulation range, where the oscilaltor is frequency-modulated, or foragiven spectral range, where it is pulse modulated, either the carrier orsome strong side band will coincide with the precession frequency of thenuclei once during each modulation cycle.

If it is not so adjusted an analysis may fail to'reveal that thesought-for nuclei are present in a sample even though in point of factthey are. Thus, if an apparatus is arranged to produce a binary type ofoutput wherein the presence and absence of a given signal, such as ofthe ringing of a bell, are intended to indicate respectively thepresence or absence of the nuclei, the latter will actually be anambiguous, and therefore unreliable, indication since it may mean either(as intended) the absence of the nucleior simply that there is amaladjust' 2,912,641 Patented Nov. 10, 1959 ment of the oscillator whichrenders the apparatus incapable of producing the signal despite theirpresence in the sample. Accordingly, it is one object of this inventionto provide for signalizing maladjustments of the oscillator and/or forautomatically maintaining an appropriate adjustment thereof even if theapparatus is at a very remote location and out of sight with respect tothe operator, e.g., at the end of a long cable deep within a bore hole.

Accordingly it is another object of this invention to devise animprovement in certain methods of analysis based on nuclear magneticresonance by including therein an additional step of continuouslymonitoring the operativene ss of the frequency adjustment of theoscillator which provides the energy for permeating the testedsamplewith a transverse field by also exposing to the analysis an auxiliarysample known to include a certain'kind of nuclei constituting a spinsystem of known responsiveness to the method to signalize on the basisof responses obtained therefrom the existence of an operative adjustmentof the oscillator and/or to maintain such an adjustment, signalsindicative of the adjustment of the oscillator being electricallytransmittable over long conductors and therefore available even if theapparatus is at a very remote location and out of sight with respect tothe operator, e.g., at the end of a long cable deep within a bore hole,or at a distant location in a chemical processing system being monitoredthrough nuclear magnetic analysis techniques.

In general the attainment of these objectives is made 'possible byproviding the test apparatus with an auxiliary detector head containinga sample which is known to include the sought for type of nuclei andwhich is both immersed in a uni-directional field of the same strengthas a principal detector head used for studying the unknown sample andpermeated with a transverse field which alternates at the same rate asthat permeating the principal detector head whereby the auxiliarydetector head will continuously produce an output signal unless theapparatus becomes maladjusted Thus an operator on seeing no signal fromthe principal detector at the same itme that the auxiliary detector isproducing a signal can rely upon the absence of signal as meaning thatthe unknown sample does not include the sought for nuclei. It iscontemplated that improved apparatus may be provided which includes theauxiliary detector head and means associated therewith for generatingerror signals representative of any driftings in the frequency of theoscillator and for applying these signals back to plished in certainvery precise ways in order to produce effects based on differences inthe relaxation times of differently bound but otherwise identicalnuclei. For example, in one suitable procedure the initial magnetizationof the sample is efiected in a precisely limited interval of time of aduration which is selected to be adequate for establishing equilibriumof protons (hydrogen nuclei) if they are bound in certain hydrocarbonsubstances, e.g., crude oil, but not for establishing equilibriumthereof if they are bound in water. The selection of a proper intervalof time usually involves making a careful compromise. For while it istrue that simply using the shortest-possible intervals will tend toemphasize the selective nature of the test, i.e., emphasize its abilityto distinguish between hydrocarbon bound and Water bound protons, suchintervals will have the disadvantage of reducing the strength of thesignal which can be derived from any given number of the sought-fornuclei therefore adversely affecting the signal-to-noise ratio andmaking it possible that small numbers thereof will not be detected atall. Moreover, for different circumstances different intervals of timeshould be selected as optimum, since the relaxation times of nuclei areactually variables depending on other factors than merely the types ofmolecules in which the nuclei may be bound, including such variablefactors as temperature, pressure, and the extent to which para-magneticmaterials may be present in or near to the sample. Thus an interval oftime which may be excellent for making tests in a portion of a bore holenear the surface of the earth, may be so unsatisfactory for a portionthereof located at great depths below the surface that signals which arenormally intended to indicate the presence of hydrocarbon bound protonsand nothing else will start to be produced in response to more Watercontained in the drilling mud. Therefore, if the apparatus is arrangedto produce a binary-type of output, the presence of the signal, as wellas its absence, can turn out to be unreliable and ambiguous. \Vhat canbe done, of course, is to reduce the time interval to make up for thefact that under the conditions which obtain at the place where the testis being made water bound protons have shorter relaxation times thanthey had at the start of the logging run. However if the interval ismade too short the result may be a complete absence of the signal notbecause of a complete absence of hydrocarbon bound protons, as intended,but because the apparatus cannot produce a signal which is observableabove the background noise in response to relatively small numbers ofsuch protons.

Accordingly it is another object of this invention to devise anadditional improvement in certain methods of analysis based on nuclearmagnetic resonance by including therein an additional step of monitoringthe operativeness or lack thereof of the apparatus for its intendedpurpose of distinguishing between nuclei which are themselves of asingle kind but are bound in diiferent kinds of molecules, i.e., aredifferently bound, to signalize to an operator its ascertained conditionof operativeness or inoperativeness, the signals being electricallytransmittable over long conductors and therefore available even if theapparatus is at a very remote location and out of sight with respect tothe operator. It is a further object to provide improved means forassuring accurate operation of nuclear magnetic resonance analysisapparatus for distinguishing between differently bound nuclei.

In general the attainment of these objects is made possible by providingthe test apparatus with 2 auxiliary detector heads each containing asample which is known to include the sought-for type of nuclei but withthe nuclei of the respective samples bound in different kinds ofmolecules; immersing both auxiliary detector heads in a uni-directionalfield of the same strength as that in which a principal detector head isimmersed; and permeating both auxiliary detector heads with a transversefield which alternates at the same rate as that permeating the principaldetector head whereby, upon proper adjustment of the apparatus such asupon proper adjustment of the interval of time over which the samplesare magnetized, one of the auxiliary detector heads will produce anoutput signal for every test cycle and the other auxiliary detector headwill not produce an output signal for any test cycle. Thus an operatoron seeing a signal from the principal detector under these circumstancescan rely upon it as meaning: (1) that the unknown sample does includethe sought-for nuclei; and (2) they are bound in a certain 'way.Moreover the operator, at times when he sees that there is no signalcoming from the principal detector head, can maximize the sensitivity ofthe principal detector by lengthening the time interval over which allof the samples are magnetized to a duration just a trifle shorter thanthat at which both auxiliary detectors would begin to produce signalscontinuously, this being readily possible, even where the operator islocated at a great distance from the detector, through the use of any ofa variety of known electrical circuit expedients.

Finally, if the operator should observe continuous signals coming fromboth detector heads regardless of how he adjusts the apparatus, he canconclude that it is useless to continue the test.

In the drawing:

Fig. 1 represents an arrangement wherein the improvements herein may beutilized in connection with a type of nuclear magnetic resonance loggingwhich can be conducted continuously with and as a part of a bore holedrilling operation;

Fig. 2 is a schematic block diagram of an embodiment of this invention;

Fig. 2a is a schematic block diagram generally similar to Fig. 2 showingstill another embodiment of the invention and,

Fig. 3 is a schematic block diagram of another embodiment of thisinvention.

The apparatus shown in Fig. 1 is of the same general type as thatdisclosed in the above-mentioned copending application Ser. No. 353,746,filed May 3, 1953 (D#35,0l5). it comprises a drilling stem 10 carryingat its lower end a bit 15 which is adapted to produce a continuous corewhich, as drilling progresses, is carried up a conduit 11 through thebody of the bit and part of the hollow interior of the bottom section ofthe drilling stern, and eventually is ejected into the mud filledannulus surrounding the stem from an orifice such as that shown at 12.The lower section of the drilling stem comprises an enlarged portion 14which is double-walled to provide an hermetically sealed annular cavitywherein components and circuit elements of the nuclear resonanceapparatus may be housed. As in the apparatus shown in the lastmentionedcopending application a tubular detector head 16 is mounted coaxiallywith the conduit 11 so that a core which is progressively developed andforced through the conduit will necessarily pass through it. Since it isthis detector head 16 that makes the most useful tests, those of unknownsamples derived from actual earth strata, it will herein be designatedas the principal detector head. In the improved. apparatus shown herein2 additional auxiliary detector heads, 18 and 19, are employed which mayalso be mounted within the hollow center of the lower section of thedrilling stem, as shown in Fig. 1, or, for that matter, in a variety ofother suitable locations near that of the principal detector head. Inany case they should normally be mounted close enough to the principaldetector head so that the known samples which they contain will beaffected in about the same ways as the unknown sample by usch influencesas temperature and pressure changes. Only in this way will the auxiliarydetectors be most reliable for these intended monitoring purposes andwill an operator be able to make suitable readjustments of the operatingconditions of the principal detector head as they are required.

In the normal use of the apparatus shown in Fig. 1 it is quite likelythat some of the drilling mud will pass through the principal detectorhead along with sample, i.e., along with the continuous core. Therefore,since the drilling mud is usually a water mix, it is important that theapparatus be adjustable to distinguish between hydrocarbon-bound andwater-bound protons and to continue to do so as various changes occur incertain ambient conditions at the location of the bit as drillingproceeds. And it is also for this reason that the known sample contained in one of the auxiliary detector heads 18 should contain waterbound protons. Preferably, this known sample should be carried in asealed container so that its essential character will be preserved eventhough it is continuously immersed in a moving column of drilling mud.To this end it may eitherxbe sealed into a hollow opening which extendsthrough the tubular detector head, e.g., by being cemented therein, or,as as alternative, the entire head, with the sample merely insertedwithin said hollow opening (and even loosely if this be desirable forany reason) may be placed within a sealed envelope. However, thechanging conditions which will unavoidably influence the unsealedunknown sample and therefore must be made to similarly influence theknown monitoring samples will include some, e.g., changes in hydrostaticpressure and contamination with para-magnetic substances, which, unliketemperature changes, may not ,bej'eflective through a sealed envelope ifit is rigid and/ or totally encloses the detector head. Accordingly,where it is desiredto duplicate at the known samples changes inhydrostatic pressure which are exerted on the unknown sample, thesesamples may be enclosed within flexible envelopes such as plasticenvelopes or metal envelopes having sylphon type side walls, and, whereit is desired to permit the circulation of some of the drilling mudthrough the auxiliary detector heads so that any contaminants which maybe intermixed therewith may similarly influence all of the samples, theknown samples may be placed directly within envelopes which are smallenough to be loosely fitted within the tubular auxiliary detector heads.However, where this is done with apparatus of the kind shown in Fig. 1it would be best to relocate the auxiliary detector heads so that themud which will circulate through them will be the mud containing themost recent cuttings from the formation, i.e., mud moving up- .wardaround the outside of the bottom section of the Y drilling stem ratherthan the downward moving column of mud contained centrally within it. Tothis end appropriate open recesses may be formed in the outer surface ofthe lower section into which the auxiliary detector, heads may bemounted or, where it is desired to cover over the detector heads, e'.g.,to protect them, and conduits, not shown, may be provided for permittingsome of the mud to be forced by the rotation of the stem into andthrough the auxiliary detector heads from the annulus which surroundsthe outside of the stem immediately above the bit.

, From the foregoing and for reasons already explained above it will beapparent that the known sample which is to be used in the otherauxiliary detector head 19 should contain the sought-for nuclei bound ina sought-for kind ,of molecule (rather than in water), e.g., it shouldcontain hydrocarbon-bound protons.

The apparatus shown in Fig. 2 comprises a principal detector head 16 and2 auxiliary detector heads 18 and .19. Each of these detector headscomprises an RF transmitter coil 21, 22 and 23 respectively, all ofwhich are fed from a common oscillator 24. To the end that it .can beconveniently adjusted automatically and/ or from aremote location, theoscillator 24 may be preferably be .of a type which can be tuned byvarying the magnitude of a direct potential which is applied to somepart thereof. For example, it may comprise a reflex kylstron whoseoperating frequency can be varied by varying its D.C. reflector voltage;or a magnetron which can be frequency modulated and/ or tuned bymodulating or readjusting an appropriate parameter of an electron beamprojected through one of its cavities; or a tank circuit associated witha hard triode reactance tube or a gas diode in any of a number of knownsuitable ways whereby an effective reactance contributed to the tankcircuit by the dis- 7 charge device and thereby the anti-resonantfrequency can be varied by varying a DC. voltage applied thereto.

Test apparatus according to this invention should preferably comprisemeans for flooding the detector head 16, 18 and 19 with uni-directionalmagnetic fields of substantially equal flux densities, such as 3separate similarly wound electro-magnets 26, 27, 28, or if preferred asingle equivalent common magnet with a suitable lowrehactance circuit todistribute the flux to all three detector heads, As in the embodimentshown herein, the magnets 26, 2-7 and 28 maybe fed in parallel withpulses of direct current from a-D.C. pulser 30. The pulser 30 maycomprise either an arrangement including: (1) a source of directcurrent; (2) an electronic switch for periodically connecting it to themagnets 26, 27 and 28 for measured intervals of time; and (3) a pulsegener-v ator for actuating the electronic switch, or, if preferred, anarrangement including: (1) a pulse generatorvand (following it) (2)enough stages of amplification to raise the amount of power contained ineach of the pulses to the necessary level for energizing the magnets'26-28. In either case a pulse generator should 'be used. A variety ofsuitable pulse generators (and even entire D.C. pulsers) are known to.the art, many of them having been developed for the modulation of radartransmitter tubes. Therefore for purposes herein it would seem to beunnecessary to describe these components in further detail. Onerequirement which should be met is that the pulser 30 can beconveniently controlled from a remote location, e.g., by the applicationof a variable DC. voltage to one of its elements to change the durationof the pulses which it produces. v

The Fig. 2 apparatus comprises a component, herein represented by block31 which is adapted to provide at least two control D.C. voltages whichan operator may vary in magnitude at will. One of these voltages isapplied to the pulser 30, over a conductor 32, to control the durationsof the pulses which it feeds to the magnets 2628 and the other isapplied to the tunable oscillator 24, over a conductor 33, for adjustingits operating frequency. The lines used in Fig. 2 to represent theconductors 32 and 33 include dotted portions to indicate that theseconductors may be of any length necessary to permit the source 31 to beremotely located with respect to the rest of the apparatus. Thus in apractical embodiment of this invention most of the components andcircuit elements shown in Fig. 2 may be located in the above-mentionedannular cavity in the bottom section of the drilling stem at the sametime that the source 31 is located on the earths surface at the topofthe bore hole.

Each of the detector heads 16, 18' and 19 comprises a respectivereceiver pick-up coil (35, 36,37) and'an associated receiver (38, 39',40) .to'whose input terminals the coil is connected. As is known it ispossible to make nuclear magnetic resonance tests by either theso-called absorption method in which attainment of the condition ofnuclear resonance can be detected through a change which it aflects uponthe input impedance of the transmitter coil or the so-called inductionmethod in which it can be detected through interception ofa signalcomprising a subsequent retransmission by the spin sys tem of part ofthe RF. energy which it absorbs upon attainment of resonance. While theapparatus shown in Figures 1 and 2 embodies the induction method it isto be understood that the present improvements may be equally wellapplied to other nuclear magnetic resonance types of apparatus.Although, it is believed unnecessary to disclose in equal detail hereinjust how the present teachings would be applied to all such otherapparatus, since those skilled in the art will be readily able to do soonce they have understood the nature of the present improvements;nevertheless an additional embodiment of the invention, as related tothe absorption method, is illustrated in Figure 3.

As is known, each of the receivers 3840 will normally comprise one ormore stages of RP. amplification to increase the amplitude of the RF.signal which reaches it from its associated pick-up coil; adetector toextract the modulation envelope of the RF. signal, e.g., low frequency(audio and/ or video) components thereof; and a low frequency amplifier.Depending upon whether, on the one hand, (1) the range over which theoscillator 24 is modulated is greater than the pass band of the receiveror, on the other hand, (2), its modulationrange is not greater or it ispulse modulated, the receiver may be (I) automatically tuned back andforth over an appropriate band of frequencies in synchronism with themodulation of the oscillator 24, or (2) it may be tuned to a fixedfrequency band. While all of the receivers 38-40 are adapted to transmittheir output signals to indicating and/ or recording devices 42, 43, 44,which will normally be positioned at the head of the bore hole at ornear the location of the control voltages source 31, one of them, namelythe receiver 49 is particularly adapted to produce a control voltage,i.e., a stop trigger, which serves a useful purpose in an arrangementfor automatically controlling the frequency of the carrier generated bythe oscillator 24. The arrangement in question comprises: (1) a squareWave generator 46 which is adapted to respond to a start trigger tostart to produce a voltage square wave and to respond to the stoptrigger to stop producing it; (2) an integrator 48 for converting theoutput of the square wave generator into a DC. voltage which may beapplied to the tunable oscillator to control its frequency automaticallyand thus supplement the manual control thereof which is effected byapplication of another DC. voltage to the oscillator over the conductor33. The start triggers are applied to the generator 46 from one of twooutputs of a modulator 50. This modulator has the principal function ofcyclically tuning or frequency modulating the oscillator, e.g., byapplying a train of saw-tooth voltages thereto from one of its outputsto cause the oscillators center frequency periodically to move initiallyat a relatively slow rate in one direction across a predeterminedfrequency band in a substantially linear manner and then to move backacross the band in the opposite direction at a relatively very fast rateto the starting frequency. In addition the modulator should be adaptedto provide at its second output a start trigger pulse at and insynchronism with the start of each saw tooth voltage. Since a largevariety of suitable ways of doing this are available in accordance withthe prior art it is believed to be unnecessary to describe any of themin detail herein. However, by way of example it is noted that onesuitable way would be to differentiate a square wave, which may beemployed in the modulator, according to one widely preferred practice,for turning on and off a switch tube which in turn is used to controlthe chargings and dischargings of a saw tooth generating RC. circuit,and to utilize as the start trigger the transient which will be producedby diiferentiation of the leading edge of said square wave. Similarly,it may be found expedient to include a differentiator in the circuitryof the receiver 40 in order to derive a sufficiently short and steeptransient from the signal which passes through it upon each attainmentof nuclear resonance to serve as a stop trigger.

It is apparent from the foregoing that by use of the improvementsdisclosed herein it is possible to avoid all of the ambiguities referredto above. Thus so long as signals continue to be present at theindicator :4 of the auxiliary receiver 40 an absence of signal at theindicator 42 of the principal receiver 33 will reliably mean thatsubstantially no protons are present in the unknown sample and cannotsimply be a manifestation that the oscillator 24 is out of adjustment,and so long as no signals are present at the indicator 4-3 of theauxiliary receiver 39 the presence of signals at the indicator 42 willmean that protons are present in the unknown sample and, in most borehole logging situations, that most likely they are hydrocarbon-bound,and it cannot mean that the protons may be water-bound.

If desired the apparatus shown herein can be simplified by eliminatedthe auxiliary detector head 19, the receiver 41 the indicator 44, thesquare Wave generator 46, and the integrator 48 and yet by operating heremaining portion in such a way that from timeto-time the auxiliarydetector head is used for monitoring the 8 oscillator adjustment whilethe logging operation is tem porarily suspended this can be done withoutsacrificing any more than the automatic frequency control feature. insuch an operation of a thus-simplified apparatus periodic spot checkupsand manual re-adjustments of'the oscillator are made by temporarilymanually increasing the duration of the DC. pulses produced in thepulser 3t and the manually re-adjusting the frequency of the oscillator,if this be necessary, until signals appear at th indicator 43.

The apparatus of Fig. 2a is similar to that shown in Fig. 2 andcorresponding elements are identified with the same reference numerals.This apparatus differs from that of Fig. 2 in that the control signalderived from the output of the integrator 48 is applied to the controlvoltage source 31 in order to provide an arrangement whereby the DC.pulser 30 may be controlled as a function of the monitor signal, as wellas the tunable oscillator 24.

The apparatus of Figure 3 comprises means for producing auni-directional magnetic field, shown as a permanent magnet having apair of spaced poled pieces till-162 defining an air gap. Within thefield of the magnet between its pole pieces 191-102 and perpendicular toits magnetic axis there is provided a principal detector head or coil103 together with first and second auxiliary detector coils lu l-1&5.The principal detec tor coil 1113 and the secondary detector coils104-105 are all positioned in substantially the same plane, transverseto the magnetic flux provided by the magnet 100 in order to assure thateach resides in a region of magnetic fiux that is substantiallyidentical with that of the other two. Each of the detector coils 163-105is provided with a respective sample container 106-108 which may be inthe form of a vial located within the respective coil. The principalcoil 1% is part of a resonant circuit 109. The resonant circuit 199 isconnected inan bridge 11% with a dummy resonant circuit 111 which hasthe same characteristics as the first resonant circuit when the latteris not undergoing nuclear resonance. A variable RF. signal generator 112is connected through a cable to one side of the bridge between theresonant circuits 1139, 111 at a point 113. The mixing point 114 on theother side of the bridge is connected to a preamplifier 115. The sectionof the bridge between the point 114 and the resonant circuit 109 isselected to introduce a half wave shift by conventional means. By way ofexample, this may be achieved by making the electrical length of theconnection between the point 114 and the resonanat circuit 1&9 one-halfwave length longer than the electrical length of the connection betweenpoint 114 and the dummy resonant circuit 111. in the alternative, theelectrical length of the connection between the point 113 and thecircuit 109 may be onehalf wave length longer than the electrical lengthof the connection between point 113 and the circuit 111. Consequently,the signal arriving normally at the mixing point under non-resonantconditions is practically zero.

Each of the secondary detector coils 104-105 forms part of a respectiveresonant circuit 109, 1 319, 109" and has associated therewith an RF.bridge, dummy resonant circuit, and pre-amplifier, corresponding to thesimilar components associated with the principal detector coil andbearing the same reference numerals as the corresponding componentsassociated with the principal coil 1&3, but distinguished by prime anddouble prime marks, respectively. The RF. signal generator 112 is alsocoupled to one side of each of the bridges between the resonant circuitsof the respective secondary coils.

A low frequency modulating coil 116 is wound around the permanent magnet1% in coaxial relationship. It is connected through the cable to a poweramplifier 117 that receives current at a low frequency (say 30*'c.p.s.')from an LF. generator 118. The power amplifier 117 also feeds aconventional phase shifter 119 which, in

generator through the power amplifier.

turn, feeds a balanced mixer 120 (balanced modulator) with current at 30c.p.s.

The output of each of the respective preamplifiers 115, 115', :115' isselectively fed through a selector switch 121 to a communicationsreceiver 122 which, in turn, feeds a narrow band amplifier 123 (30c.p.s.), the output of which is also sent to the balanced mixer 120. j

The selector switch 121 affords means for selectively coupling one ofthe respective preamplifiers associated with a desired one of thedetector coils to the communications receiver 122.

The output of the balanced mixer 120 is fed to a recorder 124 whichproduces a record 125. If desired, an additional indication of thecondition of the R.F. bridge containing the detector coil may beobtained with a conventional cathode ray oscilloscope 126 whosehorizontal plates are driven by the amplified phase-shifted 30 c.p.s.output of the low frequency generator.

A nucleus (say a nucleus of hydrogen contained in a hydrocarbon moleculein the sample) under the influence of the field produced by thepermanent magnet, will precess about the direction of the lines of forcein the field with a frequency.

magnetic field strength Plancks Constant Magnetic moment of nucleusAngular momentum If the R.F. generator is adjusted so that its output isof value.

There should be a small phase or amplitude unbalance signal present atresonance. If amplitude unbalance is used, absorption is displayed. Ifphase unbalance is used, phase-shift or dispersion curve is displayed onthe oscilloscope employed.

If a substantially homogeneous D.C. magnetic field is produced in thesample in which nuclear resonance phenomena is measured, the referencevalue for the unbalance signal is obtained by sweeping the fieldproduced by the permanent magnet in the sample back and forth throughthe resonance value. A low frequency alternating current passed throughthe modulating coil brings this about. This current is supplied by thelow frequency Current of 30 c.p.s. is suitable for modulation or sweeppurposes and will produce a persistent image of the unbalance signal -onthe cathode ray oscillograph. The signal is placed on the verticalplates of the cathode ray oscillograph while its horizontal plates aredriven by the 30 c.p.s. current,

and the oscillograph trace, as shown in Fig. 1, is a peak 'rising from alevel background. The height of this resonance peak is a measure of theamount of resonance taking place and hence of the concentration of thehydrogen-containing compound, say a hydrocarbon, for which the apparatusis tuned.

The cathode ray oscillograph is convenient for record- 'ing purposes(but allows too much noise to interfere with the signal) and in practiceis employed primarily for adjusting the apparatus. systems may beemployed, but that illustrated is par- A large number of recordingticularly useful. Its principal elements are the narrow If a narrow F1 0put is a 30 cycle wave, the amplitude of which is a measure of the slopeof the portion of the peak in the scanning range. The 30 cycle wave isamplified in the narrow band (30 cycle) amplifier, the output of whichis a sinusoidal wave whose amplitude is still a measure of the slope ofthe absorption curve'or peak. This sinusoidal signal and the 30 cyclewave from the LP.

generator are mixed in the balanced mixer circuit, which gives a DC.output proportional to the slope.

If a substantially homogeneous D.C. magnet field is produced in thesample in which nuclear resonance phenomena is measured and the magneticfield is varied through the resonance value by varying the current inthe modulating coil wound on the magnet, a curve is traced by therecorder which is a plot of the magnitude of the unbalance signal, whichmay be either plus or minus, against themagnetic field strength. Asshown in Fig. 1, the height of the absorption peak or intensity oflineis measuredby the sum of the maximum and minimum distances from the baseline along the horizontalaxis of the record. These maximum and minimumdistances actually measure the maximum and minimum slopes of theabsorption line, but these are themselves a measure of the height towhich the absorption line rises.

The line width is indicated by the vertical distance from the maximum tothe minimum of the curvew The relaxation time can be calculated from theheight of the absorption peak by the 'method described by Bloembergen,Purcell and Pound in section IV of their article in Physical Review 73p. 679 (19.48).

The resonant frequency is, of course, obtained directly from the settingof the variable R.F. signal generator.

The hydrogen nuclei (protons) in pure water at room temperature have arelatively long relaxation time (about 2 seconds) and those in typicalcrude oil at room temperature have a relatively short relaxation time(of the order of 0.1 second or less). Under the conditions of absorptiondynamic equilibrium is obtained. The rate at which energy is absorbedfrom the R.F. coil depends on the rate at which excited nuclei arede-excited. Hence, the short relaxation time characteristic of thehydrocarbons allows a greater energy absorption and thereby a strongersignal.

The presence of paramagnetic material in the'sample, tends to decreasethe relaxation time by increasing the coupling of the lattice to thenuclear moment, say to that of protons in Water. If the sample be anearth formation, for example, the paramagnetic salts in the formationare ordinarily soluble in water but not in crude oil. Hence the presenceof such salts tends to reduce the relaxation time in water, but they donot appreciably affect the relaxation time in hydrocarbons. Inconsequence, the difference in observed relaxation time for Water andhydrocarbon proton magnetic mo- -ments Will become smaller, and it willbecome more diflicult to get an indication of the presence ofhydrocarbons in the sample as the concentration of paramagneticmaterials in or adjacent thereto increases. Free oxygen is paramagneticas are iron, cobalt, nickel, chromium, copper, manganese and their ions,iron ammonium alum and potassium ferricyanide.

auxiliary detector heads with the nuclei of the respective auxiliarysamples being bound in different kinds of mole- 'cules. Since theprincipal and two auxiliary detector heads are immersed in. a commonuni-directional field of the same strength and permeated with a commontransverse field, the samples of the two auxiliary coils may be employedas monitor samples. The resonance condiilil switch to couple the desireddetector signal to the communication receiver and thus to the recorderand the oscilloscope for observation. Thus, an operator may compare anindication received from the principal detector with one or the other ofthe auxiliary samples in order to ascertain whether or not the responseof the principal detector is a valid indication of the presence of thesoughtfor nuclei in the unknown sample of the principal detector. It isalso possible through use of the monitor samples to adjust the apparatussuch that a positive, i.e., definite signal is obtained from a selectedmonitor sample in order that the same field conditions may be applied tothe unknown sample to determine whether or not identical nuclei oridentically bound nuclei are present therein.

As in the case of the apparatus shown in Figure l of the aforementionedapplication S. No. 368,547, the apparatus of Figure 3 may beincorporated in a well logging instrument or in the actual drillingequipment so that the logging head is carried close to the bit as thelatter penetrates into the ground. In this fashion, the well is loggedas it is drilled and hydrocarbon-containing structures are detected assoon as they are encountered. In still another procedure, the logginghead is held stationary at a series of levels in the bore. At eachlevel, the RF. frequency to the detector coil is varied over asubstantial range and the frequencies corresponding to the maximumabsorption signal at each level is noted. in this fashion a log isobtained in which frequency for maximum absorption signal is plottedagainst well depth. Such logs, taken in different Wells in the samefield, are useful for correlation purposes.

The foregoing procedure may be varied by periodically varying orsweeping the frequency of the applied RF. wave over a predeterminedrange as the logging head is drawn continuously along the well bore.Again, maximum absorption signals are observed and correlated with welldepth, the result being a log which is useful for correlation purposes.

The location at which nuclear resonance occurs is determined by thestrength of the DC. magnetic field and the frequency of the R.F. field.Thus, if the logging head is held stationary at selected levels in thewell bore, the depth of invasion of the formations by the drillingfluid, acidizing solutions, etc., can be measured by varying thestrength of the DC. magnetic field over a wide range. In this fashionlogs are obtained showing the characteristics of the formations alongplanes disposed perpendicularly with respect to the well bore.

In the apparatus of Fig. 3, the signal indicating the attainment of acondition of nuclear resonance adjacent the logging head is the resultof absorption of energy from the REF. circuit including the detectioncoil, the process having been designated as one of nuclear absorption.The attainment of the condition of nuclear resonance may also beindicated by a phase shift of the signal in the energizing RP. circuit(nuclear dispersion).

As will be understood by those familiar with the art, the term uni-, asused herein with respect to the direction of a magnetic field, is notintended to mean that the direction of every part of the field is thesame, though in some instances this may incidentally be true, but ratherthat the direction of any part thereof, such as the direction of one ofits so-called flux lines remains the same over time, as will be true inthe case of a field which is produced by a direct current electro magnetor a permanent magnet as contrasted to a field which is produced by analternating current electro magnet. Accordingly, it is intended that thefringing field surrounding a permanent bar magnet is within the purviewof the term uni-directional magnetic field as it is used herein and thatsimilarly a substance which is immersed in said field will beuni-directionally magnetized in accordance with the meaning intended forthis language as it is used in the specification and the claims.

It is to be understood that the principles of the inven- 12 tion may beapplied to various types of analysis by'means of nuclear magneticresonance principles. For example, the principles of the invention maybe employed in laboratory analysis and chemical processing plantmonitoring of chemical processes, as well as logging of bore holes.

Obviously many modifications and. variations of the invention ashereinabove set forth may be made without departing from the spirit andscope thereof and therefore only such limitations should be imposed asare indicated in the appended claims.

I claim:

1. In a method for non-destructively analyzing a sample under certainambient conditions by taking steps to induce nuclear magnetic resonancetherein the improvement of including an additional step ofsimultaneously under said ambient conditions inducing nuclear magneticresonance in a monitoring sample known to contain nuclei having certainpredetermined characteristics in order to obtain information relating tothe method followed in analyzing the first mentioned sample.

2. A method for non-destructively analyzing a sample comprising thesteps of: equally magnetizing unidirectionally constituent nuclei withinboth said sample and an additional monitoring sample known to containnuclei having certain predetermined characteristics; permeating each ofthe samples with a magnetic field whose orientation is transverse tothat of its uni-directional magnetization and which periodicallyreverses in direction at a rate substantially equal to the gyromagneticprecession rate of said nuclei of known characteristics under suchunidirectional magnetization; and detecting a relationship between theprecessings of said last-mentioned nuclei and the rate of said periodicreversals in direction to monitor the operativeness of the method withrespect to the firstmentioned sample.

3. A method as in claim 2 in which the last-mentioned step comprisesascertaining whether a condition of resonance is effected between theprecessings of the nuclei of known characteristics and said rate ofperiodic reversals.

4. A method for non-destructively analyzing a sample comprising thesteps of: equally magnetizing uni-directionally constituent nucleiwithin both said sample and two additional monitoring samples knownrespectively to contain nuclei which are of a single kind but are boundin different kinds of molecules; permeating each of the samples with amagnetic field whose orientation is transverse to that of itsuni-directional magnetization and which periodically reverses indirection at a rate substantially equal to the gyromagnetic precessionrate of said single kind of nuclei under such uni-directionalmagnetization; and detecting respective relationships between the rateof said periodic reversals and the precessings of nuclei of said singlekind contained in the respective monitoring samples to obtaininformation as to the operativeness of the method with respect to thefirst-mentioned sample.

5. A method as in claim 4 in which said last-mentioned step comprisesascertaining whether conditions of resonance are respectively effectedbetween said periodic rate and the precessings of nuclei of said singlekind contained in the respective monitoring samples.

6. A method as in claim 4 in which the sample to be non-destructivelyanalyzed comprises an earth constituent obtained in geophysicalprospecting and the analysis is directed at ascertaining whether thesample contains hydrocarbon bound protons and in which one of themonitoring samples comprises water-bound protons and the other compriseshydrocarbon-bound protons.

7. A method for non-destructively analyzing a succession of samples toascertain whether any thereof contain nuclei having certainpredetermined characteristics comprising the steps of: equallymagnetizing uni-directionally constituent nuclei within respective onesof said samples taken one at a time as well as constituent'nuclei ofatleast one additional monitoring sample known to contain nuclei havingsimilar characteristics; permeating each of the magnetized samples witha magnetic field whose orientation is transverse to that of itsuni-directional magnetization and which periodically reverses indirection at a rate substantially equal to the gyromagnetic precessionrate of said nuclei of known characteristics when under suchuni-directional magnetization; and detecting a relationship between therate of said periodic reversals in direction and the precessings of saidnuclei of the monitoring sample to adjudge the operativeness of themethod with respect to individual ones of the first-mentione samples. i

8. A method for non-destructively analyzing a sample comprising thesteps of: equally magnetizing unidirectionally constituent nuclei withinboth said sample and an additional monitoring sample known to containnuclei having certain predetermined characteristics; permeating each ofthe samples with a magnetic field Whose orientation is transverse tothat of its uni-directional magnetization and which periodicallyreverses in direction at ,a rate which is variable over a rangeincluding the rate of gyromagnetic precession of nuclei having saidknown characteristics when they are under such uni-directionalmagnetization; seeking the presence of a detectable relationship betweensaid periodic reversals and the precessings of said nuclei of knowncharacteristics in the monitoring sample; and varying the rate of saidreversals during said seeking until said relationship is observed toexist.

9. A method as in claim 8 in which said detectable relationship is acondition of resonance between said precessings and said rate.

10. In a method of non-destructively analyzing a number ofearth-constituent increments extending along a geophysical reference,such as along the length of a bore hole, to ascertain whether anythereof contain a certain predetermined kind of nuclei, which methodcomprises the steps of magnetizing uni-directionally each of saidincrements individually and simultaneously permeating it with a magneticfield whose orientation is transverse to that of its magnetization andwhich periodically reverses in direction at a controllable rate, theimprovement comprising the steps of: equally magnetizinguni-directionally, during the magnetization of one of said incrementslocated in one region along said reference, a monitoring sample which isknown to contain nuclei of said certain kind and is similarly located;permeating the sample during its magnetization with a magnetic fieldwhose orientation is transverse to that of its magnetization and whichperiodically reverses in direction at the same controllable rate as thatof the periodic reversals of the first-mentioned transverse field;adjusting said controllable rate until a condition of resonance iseffected between it and the gyromagnetic precessings of at least some ofthe nuclei of said certain kind in said sample; and seeking the presenceof a similar condition of resonance between said rate and that ofgyromagnetic precessings of nuclei contained in the increment then beingmagnetized.

11. An improvement as in claim 10 which comprises the further steps ofsubsequently individually magnetizing uni-directionally a number of theothers of said increments located in said region along said reference;permeating each of said other increments, during its magnetization, witha magnetic field whose orientation is I transverse to said magnetizationand which periodically reverses in direction at the adjusted rate ofsaid periodic reversals; seeking the presence of a similar conditionbetween said rate and that of gyromagnetic precessings of nucleicontained in each of said other increments as it is magnetized andpermeated; relocating the sample in another region along said reference;and re-adjusting, if necessary, the adjusted rate for said periodicreversals by again applying the steps of magnetizing, permeating andadjusting to said sample in its relocated position in said new region;and thereafter applying the steps of magnetizing, "ermeating and seekingto still others of said incre- 14 ments which are located in saidlast-mentioned region, the step of permeating being effected with atransverse field whose periodic reversals are at the re-adjusted rate.

12. A method as in claim 10 which comprises the further steps ofsubsequently individually magnetizing uni-directionally a number of theothers of said increments; permeating each "of said increments duringits magnetization with a magnetic field whose orientation is transverseto said magnetization and which periodically reverses in direction atthe adjusted rate of said periodic reversals; and seeking the presenceof a similar condition between said rate and that of gyromagneticprecessings of nuclei contained in each of said other increments as itis magnetized and permeated, and in which the steps of magnetizing andpermeating said sample are applied during the application of the stepsof magnetizing, permeating and seeking to each of said increments, andwhich comprises the further additional steps of: equallyuni-directionally magnetizing a second monitoring sample known 'toinclude nuclei which while of said certain kind are difierentlymolecularly bound than those contained in the first-mentioned sample;permeating the second sample with a magnetic field which is transverseto its uni-directional magnetization and periodically reverses indirection at the adjusted rate of said periodic reversals;

-and under these conditions and during the magnetization of eachincrement determining whether a condition of resonance between saidadjusted rate and the precessings of nuclei of said certain kind ispresent in one of the samples and absent in the other to thereby monitorthe operativeness of the method to selectively adjudge, by attainment ofa detectable condition of nuclear magnetic resonance thereof, thepresence in an increment under magnetization of nuclei of said certain.kind only if they are bound in a predetermined way.

13. A method as in claim 12 wherein in the application of the steps ofmagnetizing, permeating and seeking with respect to each of saidincrements said equal magnetization of the increment and of each of saidsamples is accomplished over a time interval which is controllable inlength to render the method operative for selectively adjudging thepresence in an increment of nuclei of said certain kind only if they arebound in a predetermined way.

14. Nuclear magnetic resonance testing apparatus for determining whetherselected nuclei are present in a specimen sample undergoing examination,comprising means for separately and simultaneously magnetizing saidspecimen sample and at least one separate monitor sample containing saidselected nuclei, with a uni-directional field, both said samples beingmaintained under common ambient conditions, means for separately andsimultaneously permeating said samples with a magnetic field theorientation of which is transverse to that of the uni-directionalfieldand which periodically reverses in the direction in which itisfitransverse thereto at a rate which is substantially equal to therate to which the selected nuclei will gyromagnetically precess in theuni-directional field, electric means for separately detecting from eachsample some eifect caused by a condition of resonance between theprecessings of said nuclei and the periodical reversings of saidtransverse field, and for producing electrical outputs separatelyindicative of said effects, a frequency modulator responsive to anoutput signal indicative of said effect upon a monitor sample, and anoscillator responsive to said modulator for modulating saidsecondmentioned means for permeating said samples.

15. Apparatus as in claim 14 comprising two separate monitor samples,one sample containing the selected nuclei bound in a molecule ofsubstance different from that of the molecule in which it is bound inthe other monitor sample.

16. Apparatus as in claim 14 which includes means l for periodicallymagnetizing the samples at repeated intervals of controlled duration.

17. Nuclear magnetic resonance testing apparatus for determining whetherselected nuclei are present in a specimen sample undergoing examinationcomprising means for separately and simultaneously magnetizing saidspecimen sample and at least one separate monitor sample containing saidselected nuclei, with a uni-directional field, both said samples beingmaintained under common ambient conditions, means, including anoscillator for generating a carrier wave, for separately andsimultaneously permeating said samples with a magnetic field, theorientation of which is transverse to that of the uni-directional fieldand which periodically reverses in the direction in which it istransverse thereto at a rate which is substantially equal to the rate atwhich selected nuclei Will gyromagnetically precess in theuni-directional field, electric means for separately detecting from eachsample some effect caused by a condition of resonance between theprecessings of said nuclei and the periodical revcrsings of saidtransverse field and for producing electrical outputs separatelyindicative of said effects, a frequency modulator responsive to anoutput signal indicative of said effect upon a monitor sample, and meansfor operatively coupling said modulator to said oscillator.

18. Apparatus as in claim 17 in which the oscillator is automaticallyadjusted to operate at least momentarily at the precession frequency ofsaid selected nuclei.

19. Nuclear magnetic resonance testing apparatus for determining whetherselected nuclei are present in a specimen sample undergoing examinationcomprising unidirectional magnetic means for simultaneously subjectingthe specimen sample and at least one separate monitor sample containingsaid selected nuclei to a unidirectional magnetic field under commonambient conditions, means for simultaneously subjecting said two samplesto an alternating field the orientation of which is transverse to thatof the uni-directional field and which periodically reverses in thetransverse direction, the combination of the means for producing saiduni-directional field and said transverse field being adapted andarranged to produce a predetermined relationship between the strength ofthe uni-directional field and the frequency of the alternating fieldsuitable to induce a condition of magnetic resonance in said selectednuclei, electric means for separately detecting the nuclear magneticresonance condition of the respective samples, means for producing acontrol signal in response to the signal detected from at least one ofsaid monitor samples, and means for applying said control signal to atleast one of said magnetic field producing means in order to maintainthe predetermined relationship between the intensity of theuni-directional field and the frequency of the alternating field.

20. Apparatus as in claim 19 wherein the control signal is applied tothe alternating field producing means in order to control the frequencyof the alternating field.

21. Nuclear magnetic resonance testing apparatus for i6 determiningwhether selected nuclei are present in a specimen sample undergoingexamination comprising unidirectional magnetic means for simultaneouslysubjecting the specimen sample and at least one separate monitor samplecontaining said selected nuclei to a uni-directional magnetic fieldunder common ambient conditions, means for simultaneously subjectingsaid two samples to an alternating field the orientation of which istransverse to that of the uni-directional field and which periodicallyreverses in the transverse direction, the combination of the means forproducing said uni-directional field and said transverse field beingadapted and arranged to produce a predetermined relationship between thestrength of the uni-directional field and the frequency of thealternating field suitable to induce a condition of magnetic resonancein said selected nuclei, electric means for separately detecting thenuclear magnetic resonance condition of the respective samples, meansfor producing a control signal in response to the signal detected fromat least one of said monitor samples, and means for employing saidcontrol signal to maintain the predetermined relationship between theintensity of the uni-directional field and the frequency of thealternating field.

22. Nuclear magnetic resonance testing apparatus for determining whetherselected nuclei are present in a specimen sample undergoing examinationcomprising unidirectional magnetic means for subjecting the specimensample and at least one separate monitor sample containing said selectednuclei to a uni-directional magnetic field under common ambientconditions, means for subjecting said two samples to an alternatingfield the orientation of which is transverse to that of theuni-directional field and which periodically reverses in the transversedirection, the combination of the means for producing saiduni-directional field and said transverse field being adapted andarranged to produce a predetermined relationship between the strength ofthe uni-directional field and the frequency of the alternating fieldsuitable to induce a condition of magnetic resonance in said selectednuclei, electric means for separately detecting the nuclear magneticresonance condition of the respective samples, means for producing acontrol signal in response to the signal detected from at least one ofsaid monitor samples, and means for employing said control signal tomaintain the predetermined relationship between the intensity of theuni-directional field and the frequency of the alternating field.

References Cited in the file of this patent UNITED STATES PATENTSHershberger Mar. 18, 1952 Levinthal Oct. 25, 1955 Andrew: NuclearMagnetic Resonance, Cambridge Press 1955.

Notice of Adverse Decision in Interference In Interference No. 91,817invclving Patent No. 2,912,641, R. J. Ruble, Analysis techniques basedon nuclear magnetm resonance, final judgment adverse to the patentee wasrendered Sept. 1, 1964, as to claims 2, 3, 8 and. 9.

[Oflicial Gazette October 27', 1964.]

